Class 12 Biology Half Yearly Question Paper 2025 Answer Key

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Class 12 Biology Half Yearly Question Paper 2025 Answer Key (NCERT Based)

Instructions :
• All questions are compulsory.
• Questions (1-9) carry 1 mark each.
• Questions (10-12) carry 2 marks each.
• Questions (13-14) carry 3 marks each.
• Question (15) case study, carry 4 marks.
• Questions (16-17) carry 5 marks each.

1. Feathery stigma occurs in :
(A) Pea
(B) Wheat
(C) Datura
(D) Caesalpinia
Answer – (B) Wheat

2. Increased IMR and decreased MMR in a population will :
(A) cause rapid increase in growth rate
(B) result in decline in growth rate
(C) not cause significant change in growth rate
(D) result in an explosive population.
Answer – (B) result in decline in growth rate

3. The most accepted line of descent in human evolution is :
(A) Australopithecus → Ramapithecus → Homo sapiens → Homo habilis
(B) Homo erectus → Homo habilis → Homo sapiens
(C) Ramapithecus → Homo habilis → Homo erectus → Homo sapiens
(D) Australopithecus → Ramapithecus → Homo erectus → Homo habilis → Homo sapiens
Answer – (C) Ramapithecus → Homo habilis → Homo erectus → Homo sapiens

4. Which was the last human chromosome to be completely sequenced?
(A) chromosome one
(B) chromosome 11
(C) chromosome 21
(D) chromosome X
Answer – (A) chromosome one

5. State the role of the epididymis in male fertility.
Answer – The epididymis stores the sperm for maturation and transports it to the vas deferens during ejaculation.

6. What are coacervates?
Answer – Coacervates are colloidal droplets formed by the aggregation of hydrophobic proteins and lipids.

7. …………….. enzyme joins the small fragment of DNA of a lagging strand during DNA replication.
Answer – DNA ligase

8. Smack is a drug obtained from the ……………
Answer – opium poppy (Papaver somniferum)

9. Assertion (A) : In transcription, only a segment of DNA and only one of the strands is copied into RNA.
Reason (R) : The process of replication, once set in, duplicates the total DNA of an organism.
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
(B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
(C) Assertion is true, but Reason is false.
(D) Assertion and Reason both are false.
Answer – (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.

10. Copper ion releasing IUDs are more efficient than non medicated methods. Why?
Answer – Copper IUDs are more efficient than non-medicated methods because :
(i) Copper ions reduce sperm motility and fertilizing ability.
(ii) They increase uterine phagocytosis of sperm, preventing fertilization.

11. What are multiple alleles? Give an example.
Answer – Multiple alleles are more than two alternative forms of a gene that occupy the same locus, though an individual carries only two of them. e.g. The ABO blood group in humans, controlled by three alleles IA, IB, and i.

12. Differentiate divergent evolution from convergent evolution.
Answer : Divergent Evolution – Species with a common ancestor evolve different traits to adapt to different environments, becoming dissimilar. e.g. Darwin’s finches- different beak shapes.
• Convergent Evolution – Unrelated species evolve similar traits due to similar environments, becoming alike in function or structure. e.g. Wings of bats and birds.

13. (a) Why is bagging of emasculated flowers essential during hybridization experiments?
Answer – Bagging of emasculated flowers is done to prevent unwanted pollen from reaching the stigma, ensuring controlled cross-pollination.

(b) Mention the cells of the mature pollen grain.
Answer – Vegetative cell (tube cell) and Generative cell (two male gametes)

(c) Give the scientific name of the plant that has the viability record of 10,000 years.
Answer – Lupinus arcticus

14. What are autoimmune diseases? Give two examples.
Answer – Autoimmune diseases are disorders in which the immune system, instead of protecting the body, mistakenly attacks its own healthy cells, tissues, or organs, leading to damage and impaired function. e.g. Type 1 Diabetes (where insulin-producing cells in the pancreas are destroyed) and Rheumatoid Arthritis (where the immune system targets the joints, causing pain, swelling, and stiffness).

15. CASE STUDY : Mendelian disorders are mainly determined by alteration or mutation in the single gene. These disorders are transmitted to the offspring on the same lines as we have studied in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis. Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc.
Questions :
(i) Which of the following disorders is also called the Royal disease?
(A) Colour blindness
(B) Haemophilia
(C) Sickle cell anaemia
(D) Alzheimer’s disease
Answer – (B) Haemophilia

(ii) Which of the following genotypes and phenotypes in a man may be the correct result of aneuploidy in sex chromosomes?
(A) 22 pairs + Y females
(B) 22 pairs + XY females
(C) 22 pairs + XXY males
(D) 22 pairs + XXXY females
Answer – (C) 22 pairs + XXY males

(iii) Cystic fibrosis is an autosomal recessive genetic disorder. What are the chances that the child would have the disease if any one of the parents (either mother or father) is a carrier of the faulty cystic fibrosis gene (Cc)?
(A) 100 per cent
(B) 50 per cent
(C) 25 per cent
(D) 0 per cent
Answer – (D) 0 per cent

(iv) If the father in a family has disease while the mother is normal, the daughters only are inherited by this disease and not the sons. Name this type of disease?
(A) Autosomal recessive
(B) Autosomal dominant
(C) Sex-linked recessive
(D) Sex-linked dominant
Answer – (D) Sex-linked dominant

16. The zygote passes through several development stages till implantation. Describe each stage briefly with suitable diagrams.
Answer – From the formation of the zygote to implantation, this flow (zygote → cleavage → morula → blastocyst → implantation).

After fertilization, the zygote undergoes a series of development stages before it implants into the uterine wall. These stages are :
(i) Zygote Formation – Fertilization occurs in the fallopian tube, where a sperm fuses with an egg to form a diploid zygote. This zygote is the first cell of a new individual and contains a complete set of chromosomes, half from the mother and half from the father.
(ii) Cleavage – The zygote undergoes rapid mitotic divisions called cleavage, producing smaller cells called blastomeres. During cleavage, the total size of the embryo does not increase; instead, the single zygote is partitioned into many cells, preparing for differentiation.
(iii) Morula Formation – After several cleavages, the embryo becomes a solid ball of cells called the morula. All cells in the morula are genetically identical. This stage resembles a mulberry, which is why it is named “morula.”
(iv) Blastocyst Formation – The morula develops a fluid-filled cavity, transforming into a blastocyst. The blastocyst has two distinct parts:
Trophoblast: The outer layer of cells that will form part of the placenta and help in nutrient exchange.
Inner cell mass (embryoblast): A group of cells inside the blastocyst that will develop into the embryo. This stage also initiates cell differentiation, marking the start of organized development.
(v) Implantation – Around 6-7 days after fertilization, the blastocyst reaches the uterus and attaches to the endometrium, the lining of the uterus. The trophoblast cells invade the uterine lining to establish nutrient and gas exchange between the mother and the developing embryo. This process, called implantation, is essential for the continuation of pregnancy.

17. Discuss the process of translation in detail.
Answer – Translation is the process by which a protein (polypeptide chain) is made using the instructions in mRNA. Ribosomes read the codons on mRNA and bring the correct amino acids via tRNA molecules to build the protein. Translation takes place in the cytoplasm on ribosomes.
• Stages of Translation :
(i) Activation of Amino Acids – Before translation starts, each amino acid is attached to its specific tRNA. This process is called aminoacylation or charging, is done by the enzyme aminoacyl-tRNA synthetase, and uses ATP. The resulting aminoacyl-tRNA is ready to deliver the correct amino acid to the ribosome.
(ii) Initiation – The small ribosomal subunit binds to the 5’ end of the mRNA with the help of initiation factors. The first tRNA, carrying methionine (Met), attaches to the start codon (AUG) at the P site. Then the large ribosomal subunit joins, forming a complete ribosome ready for protein synthesis.
(iii) Elongation – The next tRNA carrying the matching amino acid binds to the A site. The ribosome forms a peptide bond between the amino acid in the P site and the new one in the A site. Then the ribosome moves one codon forward (translocation): the tRNA in the P site shifts to the E (exit) site and leaves, while the tRNA in the A site moves to the P site with the growing chain. This process repeats, adding amino acids one by one.
(iv) Termination – When a stop codon (UAA, UAG, UGA) reaches the A site, a release factor binds instead of tRNA. This releases the completed polypeptide, and the ribosome, mRNA, and release factor separate, ending translation.